1. Field of the Invention
The present invention relates in general to pre-stressed concrete. More particularly, it relates to methods of making tendons which are used primarily in post-tensioning of pre-stressed concrete.
2. Description of the Prior Art
In the process of pre-stressing concrete is by the technique of post-tensioning, it is important that the tendon be free to move within the hardened concrete so that the tensile load on the tendon is evenly distributed along the length of the tendon.
Various methods have been used in attempts to assure that the tendons used for post-tensioning concrete are capable of free movement within the hardened concrete. These methods include the laying of a number of parallel strands of wire in a metal duct or plastic tube and then stretching the strands after the concrete is hardened. The duct or tube is usually filled with grease after the strands are stretched. Also, a number of parallel strands of wire have been covered with grease and then covered with spirally wound paper. In some cases the spirally wound paper is replaced by spirally wound plastic. Another alternative to the paper wrapped tendon is the cigarette wrap or lap seam tendon. Tendons have also bee produced by stuffing or pushing a greased seven wire strand into and through a previously extruded plastic tube.
U.S. Pat. No. 3,646,748 discloses a post-tensioning tendon which comprises a multiple-wire strand encased in a corrosion inhibitor in an amount sufficient to provide a circular encasement around the strand of a diameter at least two mils greater than the diameter of the strand and having a seamless plastic jacket tightly covering the encased strand. A process for making such tendons is also disclosed.
All such prior tendons, however, suffer from one or more practical deficiencies such as high manufacturing cost, low reliability and a low level of assurance of trouble-free service. The present invention provides post-tensioning tendons which are without the deficiencies of the prior art.